Nonmagnetic impurity perturbation to the quasi-two-dimensional quantum helimagnet LiCu2O2

A complete phase diagram of Zn-substituted quantum quasi-two-dimensional helimagnet LiCu2O2 has been presented. Helical ordering transition temperature (Th) of the original LiCu2O2 follows finite size scaling for less than ∼5.5% Zn substitution, which implies the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution ≥5.5% quenches the long-range helical ordering and introduces an intriguing Zn-level-dependent magnetic phase transition with slight thermal hysteresis and a universal quadratic field dependence for Tc(Zn>0.055,H). The magnetic coupling constants of nearest-neighbor J1 and next-nearest-neighbor J2 (α=J2/J1) are extracted from high temperature series expansion fitting and N=16 finite chain exact diagonalization simulation. We have also provided evidence of direct correlation between long-range helical spin ordering and the magnitude of electric polarization in this spin-driven multiferroic material.